1. Field of the Invention
This invention relates to the field of microwave synthesis of materials, particularly, ultrafine powders having an average particle size  less than 500 nm.
2. Description of the Related Art
Ultrafine metallic and ceramic powders have unique properties, and have the potential to contribute to significant advances in the field of electronics, solid lubricants, capacitors, batteries, sensors, thermal management substrates, and additives for the cosmetic and pharmaceutical industries. Ultrafine powders also find applications in optical coatings, slurries used for polishing, and in magnetic storage devices. Parts produced out of ultrafine particles/powders demonstrate improved mechanical, optical, and thermal properties. Conventionally, ultrafine powders have been produced by a variety of techniques including mechanical milling, flame pyrolysis, sol-gel, laser ablation, vapor deposition, and evaporation-condensation techniques.
Low power (1-2.5 kW) microwave generated plasmas have been used in many deposition, etching, and substrate processing operations. Low power microwave systems operate at plasma temperatures of less than 700xc2x0 C., deposition and etching chamber are traditionally made out of brass and bronze or even copper, with quartz tube lining in some cases. These chambers or applicators can withstand 700xc2x0 C. without much cooling requirements.
The application of microwaves to synthesize metallic and ceramic powders offers unique benefits, especially in producing particles of submicron size with controlled compositions and phases.
The present invention includes an apparatus and method for producing materials, preferably ultrafine powders, using microwave plasma chemical synthesis. The principle components of a microwave machine in accordance with the invention are: (1) a microwave generator, such as a magnetron, and (2) a microwave applicator. A magnetron produces microwaves by the interaction of electrons traveling in electric and magnetic fields (often referred to as xe2x80x9ccrossed fieldsxe2x80x9d). This interaction coupled with high DC voltage between the cathode and the anode results in microwaves. Microwaves thus generated are then passed through waveguides before they arrive at the head of the applicator.
Microwave applicators are devices that are designed to heat a material by exposing it to a microwave field in a controlled environment. In the present invention, the applicator is referred to as xe2x80x9cplasmatron,xe2x80x9d wherein the high energy microwave electrons ionize and dissociate the injected gas thus releasing large amounts of energy. The energy thus released is utilized to initiate a chemical reaction between the desired reactants. The interaction between the chemical species results in ultrafine powders with the desired chemical and physical characteristics. Due to rapid quenching that takes place in the reactor column the powder sizes are very fine. By controlling the diameter and length of the column, it is therefore possible to control the particle size. This apparatus can produce ultrafine powders of pure metals, such as iron, cobalt, nickel, tungsten, and rhenium; metal oxides, such as iron oxide; metal nitrides, such as titanium nitride; metal carbides; and many other ceramics, such as aluminum nitride, titanium dioxide, and aluminum dioxide. The apparatus also enables the continuous production of ultrafine particles/powders of pure metals, metal oxides, metal carbides, and metal nitrides, particularly tungsten, molybdenum, iron, cobalt, nickel, aluminum, titanium dioxide, and aluminum nitride, in contrast to the batch processes of the prior art.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the process and apparatus, particularly pointed out in the written description and claims hereof, as well as the appended drawings.
To achieve these and other advantages, and in accordance with the purpose of the invention as embodied and broadly described, the invention includes an apparatus for the microwave synthesis of materials. The apparatus includes a microwave generator, waveguides through which microwaves generated by the microwave generator passes into a plasma zone in which a plasma heats the reactants to form reaction products, a plasma gas inlet offset at an angle so that plasma gas passing through the inlet enters the plasma zone in a spiral-shaped pattern, and a reaction products collector downstream from the plasma zone for collecting the reaction products.
In another aspect, the invention includes an apparatus for the microwave synthesis of materials. The apparatus includes a microwave generator, waveguides through which microwaves generated by the microwave generator passes into a plasma zone in which a plasma heats reactants to form reaction products, a microwave damper at a side of the plasma zone away from the waveguide for damping microwaves that have passed through the plasma zone to thereby reduce the power of a reflected microwave by at least 60%, and a reaction products collector downstream from the plasma zone for collecting the reaction products.
In yet another aspect, the invention includes a method for the microwave synthesis of materials. The method includes introducing a plasma gas into a microwave applicator, introducing at least one reactant into the microwave applicator, generating a microwave in a microwave generator, directing the microwave into the microwave applicator containing the plasma gas to create a heated plasma in a plasma zone, absorbing the microwave to reduce the microwave reflection off of a surface of the microwave applicator by at least 60%, and causing a reaction in the plasma zone thereby converting at least one reactant into a reaction product.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate one/several embodiment(s) of the invention and, together with the description, serve to explain the principles of the invention.